1. Field of the Invention
The present invention relates to a biaxially oriented laminated polyester film comprising a polyester substrate layer, a thin polyester surface layer laminated on one surface of the substrate layer and optionally a thin polyester surface layer laminated on the other surface of the substrate layer, and having uniform surface roughness, lubricity, abrasion resistance and scratch resistance in the width direction of the surface layer of the film.
2. Prior Art
A biaxially oriented polyester film has excellent properties and is therefore used in various fields of magnetic tapes, electric parts, photography, metallizing and packaging. Above all, a biaxially oriented polyester film has high strength and a high elastic modulus and is therefore widely used as a base film for magnetic recording media such as a video tape, an audio tape, a computer tape and a floppy disk.
In these fields, with growing demands for high-density recording and in compliance with a higher quality in recent years, a polyester film as a base film is strongly required to have a flat surface.
However, as the flatness of a film surface increases, there will arise such problems that in the usage of magnetic tape, for example, the film has an increased coefficient of friction and it is liable to fail in running or suffer scratching. Further, when a film having a flat surface is taken up in the form of a roll in the film production, the form of a roll is liable to be markedly poor, or it is difficult to take up the film in a wood rolled form.
Moreover, for improving the productivity of a film, it is required to increase the take-up rate and broaden the film width. However, the increase in the take-up rate and the broadening of the film width involve a problem in that it is increasingly difficult to take up the film in a good rolled form.
For accomplishing the flatness of a film, it is concurrently required to overcome the above problems caused by the flattening of the film surface.
Under the circumstances, there is proposed a laminated film having different values of surface roughness between its one surface (front surface) and the other surface (back surface), i.e., a laminated film of which one surface is flat and the other surface is rough. This laminated film is advantageously used as a film of which one surface is required to be flat and the other surface may be rough, such as a base film for a magnetic tape. When a magnetic layer is formed on the flat surface of the above film and when the other rough surface is used as a running surface, the above film is excellent as a base film for a magnetic tape, since the magnetic tape can have both flatness for improving electromagnetic conversion characteristics and lubricity for improving film running properties.
In recent years, however, it is required to decrease the roughness of the rough surface, too and the "rough" surface is increasingly flattened as well.
Under the circumstances, the fluctuation of the film surface roughness becomes outstanding as differences in film properties and particularly, the fluctuation of surface roughness in the film width direction comes to cause a difference in take-up properties.
For example, in the production of a magnetic tape, one of products for which a polyester film is used, a large-length polyester film having a large width (e.g., 300 to 1,500 mm) is coated with a magnetic layer and calender-treated to prepare a large-width web film, the treated film is slit, e.g., to a width of 8 mm or 1/2 inch to prepare tens of slit tapes, and these slit tapes having a length of hundreds to thousands meters are simultaneously taken up to prepare pancake rolls. When the surface roughness of the above large-width base film fluctuates, the running surfaces of the slit tapes have different values of surface roughness.
When the slit tapes are taken up under conditions conforming to slit tapes having a relatively "flat" surface roughness, rolls of slit tapes having a relatively "coarse" surface roughness are liable to have a nonuniform side face due to the higher smoothness or slipperiness thereof. As a result, the rolled forms of the pancake rolls are poor to decrease the yield. When the side face of a pancake roll is too nonuniform, the pancake roll has a broken form and the operation may be stopped.
On the other hand, when the slit tapes are taken up under conditions conforming to slit tapes having a relatively "coarse" surface roughness, slit tapes having a "coarse" surface roughness give pancake rolls having a good rolled form. However, slit tapes having a relatively "flat" surface roughness may give pancake rolls having bumps or having an irregularly circular form or a slightly angular form because of insufficient lubricity. As a result, some pancake rolls have poor rolled forms to decrease the yield of products.
That is, when the surface roughness of a large-width film varies in the width direction, not all the pancake rolls of slit tapes obtained by slitting the large-width film have an excellent rolled form.
Japanese Laid-open Patent Publication No. 126,723/1988 discloses a process for the production of a biaxially oriented film having improved haze uniformity in the width direction, in which an unstretched film having a higher concentration distribution of inert inorganic lubricant particles in sides and a lower concentration distribution in a central area is prepared and biaxially oriented. However, the above publication neither discloses nor suggests the present invention.